I have a passive 3d monitor and nvidia GPU. When playing splitscreen in portal, I would like to set my monitor in SBS or TB mode, which is fine, but is there a way to force the aspect ratio to be correct? should I be approaching this a different way?

Bump. Im aware that source has a whole bunch of commands in the console but couldn't see anything usefull there, or on google either. Perhaps I wasn't googling correctly though. If I can do this either through nvdia or portal that would be great

Not quite sure if I get what you wanna do or whats a possible solution. Maybe this will help you anyway.

I have an AMD GPU and used a 2nd screen for splitscreen. combined both screens with eyefinity so It was treated as one big 3840x1080 "display". Now Portal 2 was showing only half a screen on each display. I had to go to steamapps/common/portal 2/portal 2 and edit splitscreenconfig.txt and adjust aspect ratio for widescreen_vertical_split from 8 by 9 to 16 by 9 so it will use the whole space.

If you're doing what I think you're doing you'd need to add some custom 32 by 9 resolution cause this setting won't stretch the image to fullscreen.